The Allegheny Plateau is a vast physiographic region in the eastern United States, recognized for its distinct landscape. It forms a significant part of the larger Appalachian Mountains system, characterized by a unique blend of elevated terrain and deeply incised valleys. It is a dissected plateau, where ancient rock layers have been uplifted and carved by natural forces over millions of years.
Geographical Extent and Defining Features
The Allegheny Plateau spans a considerable area, extending across multiple states in the eastern United States. It covers western and central New York, northern and western Pennsylvania, eastern Ohio, and northern and western West Virginia. Further south, it continues into eastern Kentucky and Tennessee, where it is known as the Cumberland Plateau. This region is a distinct section of the broader Appalachian Plateau province.
The topography of the Allegheny Plateau is a dissected plateau, featuring elevated, relatively flat uplands that are deeply cut by numerous rivers and streams, forming narrow valleys. This contrasts with the more typical rugged peaks of mountain ranges, as the plateau’s surface was once a continuous, high plain. Distinct landforms across the plateau include escarpments, such as the prominent Allegheny Front, which marks its eastern boundary.
Deep ravines and gorges, like Pennsylvania’s Pine Creek Gorge, are features carved by persistent water flow. The drainage patterns across the plateau often exhibit a dendritic (tree-like) shape. Elevations vary significantly, with glaciated northern parts having gentler relief, typically around 100 feet or less. In contrast, unglaciated areas can show relief between 200 to 400 feet, reaching over 4,000 feet near the Allegheny Front.
Geological Formation and Composition
The formation of the Allegheny Plateau began with the deposition of vast layers of sediment in ancient marine environments hundreds of millions of years ago. These sediments, sand, mud, and organic matter, accumulated horizontally over time, compacting into sedimentary rocks. The predominant rock types found across the plateau include sandstone, shale, and limestone. Coal seams are interbedded within these layers, particularly in the southern parts of the plateau, indicating past periods of lush vegetation.
Following this long period of sedimentation, the region experienced uplift during the Appalachian Orogeny, mountain-building events. Unlike the folded and faulted rocks of the Appalachian Ridge-and-Valley province, the Allegheny Plateau’s rocks remained flat-lying during this uplift. After uplift, extensive erosion by water became the primary force shaping the landscape. Rivers and streams incised into the plateau, carving the deep, narrow valleys seen today.
The northern portions of the plateau were impacted by Ice Age glaciation. Massive ice sheets advanced and retreated, scraping away loose material and smoothing terrain and resulting in lower relief compared to the unglaciated southern sections. This combination of deposition, uplift, and erosion by both water and ice created the Plateau’s dissected topography. The differential erosion of varying rock hardness, where harder layers resist erosion more than softer ones, further contributed to the formation of the complex network of ridges and valleys.
Ecological Characteristics and Biodiversity
The Allegheny Plateau supports diverse ecological communities, shaped by its topography and climate. Its forests are mixed deciduous, including central hardwoods. Forest compositions include mixed mesophytic, Appalachian oak, and northern hardwood forests, reflecting differences in elevation, moisture, and past land use. Common tree species include red maple, white oak, black oak, American beech, tuliptree, sugar maple, and various ash species. Eastern hemlock and black cherry are particularly in certain forest types.
The plateau’s varied habitats support wildlife. Mammal species include white-tailed deer, cottontail rabbits, groundhogs, and wild turkeys. Gray and fox squirrels, raccoons, opossums, red foxes, and striped skunks are also present throughout the region. Bird species are abundant; the Allegheny Front, an escarpment, is a notable location for observing migrating raptors, including Golden Eagles.
Geology and hydrology form unique habitats. Deep ravines and gorges, often with steep cliff faces, create sheltered environments and distinct microclimates. The extensive network of streams and rivers contributes to high aquatic biodiversity, though some waterways faced historical industrial pollution. Much of the original old-growth forest was cleared by logging, especially between 1890 and 1920. However, some remnants and regenerating forests continue to provide important ecological functions. Northern glaciation also formed numerous small lakes, which are absent in the unglaciated areas to the south.